1. Field of the Invention
The present invention relates to organic electroluminescent devices, in particular phosphorescent organic electroluminescent devices.
2. Description of the Related Art
One class of opto-electrical devices attracting increasing attention is that using a semiconducting organic material for light emission (an electroluminescent device) or as the active component of a photocell or photodetector (a “photovoltaic” device). The basic structure of these devices is a semiconducting organic layer sandwiched between a cathode for injecting or accepting negative charge carriers (electrons) and an anode for injecting or accepting positive charge carriers (holes) into the organic layer.
In an organic light emitting device (OLED), electrons and holes are injected into the semiconducting organic layer where they combine to generate excitons that undergo radiative decay. Various classes of organic light emitting materials are known, in particular: polymers such as poly(p-phenylenevinylene) (as disclosed in WO 90/13148), polyfluorenes and polyphenylenes; the class of materials known as small molecule materials such as tris-(8-hydroxyquinoline)aluminium (“Alq3”) as disclosed in U.S. Pat. No. 4,539,507; and the class of materials known as dendrimers as disclosed in WO 99/21935. These materials electroluminesce by radiative decay of singlet excitons (i.e. fluorescence) however spin statistics dictate that up to 75% of excitons are triplet excitons which undergo non-radiative decay, i.e. the theoretical maximum of quantum efficiency for fluorescent OLEDs is 25%—see, for example, Chem. Phys. Lett., 1993, 210, 61, Nature (London), 2001, 409, 494, Synth. Met., 2002, 125, 55 and references therein.
Accordingly, considerable effort has been directed towards producing luminescence from triplet excitons (phosphorescence) by utilising spin-orbit coupling effects in metal complexes that enable triplet excitons to undergo radiative decay. Examples of complexes investigated for this purpose include lanthanide metal chelates [Adv. Mater., 1999, 11, 1349], a platinum (II) porphyrin [Nature (London), 1998, 395, 151] and tris-phenylpyridine iridium (III) (hereinafter Ir(ppy)3) [Appl. Phys. Lett., 1999, 75, 4; Appl. Phys. Lett., 2000, 77, 904]. Fuller reviews of such complexes may be found in Pure Appl. Chem., 1999, 71, 2095, Materials Science & Engineering, R: Reports (2002), R39(5-6), 143-222 and Polymeric Materials Science and Engineering (2000), 83, 202-203.
The emissive layer of an OLED may consist of a neat film located between the anode and cathode, optionally with further charge transporting layers. In an alternative arrangement, the emissive material is provided as a dopant within a charge transporting host material. This arrangement may serve to increase device efficiency by improving charge transport and/or providing exciton transfer from the host material to the emissive material. The host-dopant arrangement may be applied to fluorescent materials as described in, for example, J. Appl. Phys. 65, 3610, 1989 or phosphorescent materials as described in the aforementioned disclosures of phosphorescent OLEDs.
The emissive layer of an OLED may be cross-linked to render it insoluble following its deposition. Cross-linking is particularly advantageous where the emissive material is soluble and may be otherwise be dissolved if further solution processing steps are undertaken.
Cross-linking may be used to form additional device layers by solution processing. For example, U.S. Pat. No. 6,107,452 discloses a method of forming a multilayer device wherein fluorene containing oligomers comprising terminal vinyl groups are deposited from solution and cross-linked to form insoluble polymers onto which additional layers may be deposited. Similarly, Kim et al, Synthetic Metals 122 (2001), 363-368 discloses polymers comprising triarylamine groups and ethynyl groups which may be cross-linked following deposition of the polymer.
Cross-linking may also be used for photolithographic patterning of an electroluminescent layer by UV cross-linking of the electroluminescent layer using a mask followed by washing of the electroluminescent layer with a solvent to remove non-cross-linked material. For example, further solution processing may be desirable in order to deposit additional device layers from solution and/or to wash away For example, Nature 421, 829-833, 2003 discloses a method of forming a full colour display by deposition of layers of red, green and blue electroluminescent polymers bearing oxetane side groups which are cross linked via a photoacid generator after deposition by exposure to the appropriate radiation. Similarly, JP 2003-142272 discloses a cross-linking of a hole transport layer, which may optionally be photopatterned, prior to deposition of the electroluminescent layer.
Thiol-ene polymers are known for use in photolithography (though not photolithography of OLEDs) —for example, see Jacobine, Radiat. Curing Polym. Sci. Technol., 1993, 3, 219-68.
Co-pending application PCT/GB 03/00899 describes use of thiol-ene polymers for photopatterning of OLEDs, in particular OLEDs comprising a host-dopant system as described above. This application describes charge transporting moieties comprising thiol or alkene groups that may be polymerised in the presence of an emissive material such as Ir(ppy)3 to form an electroluminescent layer comprising a charge transporting host polymer matrix containing the emissive dopant material within the matrix. This layer may then be subjected to solution processing such as photopatterning. Although this approach serves to provide a functioning, patterned OLED, the present inventors have found that the processing steps following deposition of the electroluminescent layer causes the efficiency of photopatterned devices made according to this approach to be relatively low.
WO 03/01616 discloses monomers of phosphorescent complexes such as tris-phenylpyridine iridium (III) bearing acrylate groups. OLEDs comprising these complexes may be formed by polymerising the acrylate groups and then solution depositing the polymer onto the OLED substrate, or polymerising the monomer after its deposition. The latter case is preferred if the degree of cross-linking in the polymer renders it insoluble. This document discloses soluble and insoluble polymers, and does not disclose further solution processing steps following deposition of these polymers.
In view of the aforementioned problem of low efficiency, in particular for devices such as photopatterned devices, it is an object of the invention to provide a method of forming an electroluminescent device comprising a host-dopant electroluminescent layer having improved efficiency.